The size of silicon wafers used in semiconductor and integrated circuit manufacturing has grown in diameter now to 450 mm and beyond. Many processes use plasmas generated from gases in clouds over the wafer surface to heat, etch, clean, rinse, and deposit materials on the silicon wafers in dozens of steps to build hundreds, and even millions, of circuits across the surface of each wafer.
As the average diameters of the silicon wafers grew over time, it became increasingly more difficult to maintain process uniformities over the entire working surface. One such difficulty was in providing a uniform discharge gas in which to induce a planar plasma. So thick gas distribution plates that resemble water showerheads were developed that included hundreds of small diameter gas flow passages to even out the discharge gas across a wide area.
Unfortunately, these gas distribution plates were unavoidably being exposed indirectly to the same processes the silicon wafers were, and so the gas distribution plates were slowly being ruined by residual etchants and deposits. Wet-washing works well for the other affected parts inside the processing chambers, but wet-washing cannot get inside the gas flow passages because they are too narrow, e.g., on the order of one millimeter.
Gas distribution plates are made from at least two alternative materials, silicon ingot and aluminum billet. Each has a particular application. Applied Materials makes aluminum showerheads for PECVD deposition processes, and these degrade with all the gas flow passages on a grid pattern being more or less equally contaminated with AlF3. Tokyo Electron Ltd (TEL) makes silicon showerheads that are used in etching processes, these degrade because plasma ions bombard the process face and sputter material onto the outer edges. This degradation is not uniform, the worst damage concentrates around the outer periphery. Refurbishing involves reconstruction, in silicon, of the lost pieces of the inside walls of the gas flow passages around the periphery. These gas flow passages are set in radial patterns.
Both aluminum and silicon gas distribution plates are very expensive to replace, so refurbishing them makes good economic sense.
These large gas distribution plates have gas flow holes that pierce completely through. These high aspect through-holes are relatively small in diameter, compared to thickness of the gas distribution plate itself, for example a 0.062″ diameter hole through a 1.2″ thick plate. Maintaining the original hole geometry of all the gas flow passages is very important.
During various PE-CVD processes involving aluminum showerheads, these holes can get eroded and clogged. Most of the volatile by-products typically produced during reactions get pumped out from the chamber through an exhaust system. But enough residue remains on the surface of the gas distribution plate and inside the gas flow passages to eventually cause trouble. Eventually the entire gas distribution plate has to be scrapped, not repaired, because there has been no conventional way to restore them.
The cost of materials and manufacturing of gas distribution plate is substantial. So it would be advantageous to increase the lifetime of gas distribution plates by protecting them from plasma chemical corrosion with special coatings. But a means has yet to have been develop where gas distribution plates can be efficiently and cost effectively refurbished. Moreover, as the size of the next generation gas distribution plates is increasing to accommodate next generation processing wafers in excess of 1.2 square meters, a solution becomes increasingly important.
What has been holding everyone back is a well-known Debye self-shielding effect that ordinarily blocks plasma beam penetration of millimeter scale passages, grids, and holes. Plasmas thus have a characteristic length, known as a Debye Length, which can be represented by,L (cm)=743(Te/ne)exp(0.5)where Te is the electron temperature in eV, and ne is the electron density in electrons/cm3.
Typical high-density atmospheric pressure ICP plasmas have a density of 10 exp(14)/cm3. High density supersonic plasma beams have a relatively low electron temperature of around 0.04 eV. Therefore the Debye Length for these can be computed to be around five millimeters. Well short of 1.2″ needed to pass through a typical gas distribution plate.
Through-holes that are shorter than the Debye self-shielding length will cause an electron congestion that impedes plasma beams from completely penetrating intact. The Debye blocking forms from a negative sheath layer that appears inside the gas flow passages and is continually charged by charged particles stripping off the plasma beams trying to penetrate. Conventional wisdom has been just to accept this limit as a given and work some other approach.
Plasma-based refurbishment systems and tools are needed more than ever that can effectively clean and restore both silicon and aluminum types of gas distribution plates to service. In spite of the Debye blocking.